A new technology is evolving using wetland plants to treat contaminated and/or polluted water which will be referred to hereafter as "waste water". The technology, as it has evolved to the present time, is to build a rectangular or square shaped basin, with the preferred design being a rectangular shaped basin whose length is several times its width. The basin bottom and sides are lined with a plastic, water impervious material. The basin is then filled with solid materials such as ordinary soil, clay, rock, gravel, sand, silt, fly ash or a combination of the above, to a depth of twelve inches to two and half feet. The nature of the solid materials may be any of the above or others; the nature of such solid materials is not germane to the present invention. Any wetland plant which can be found growing naturally in wet or damp environment such as ponds, rivers, marshes, and drainage ditches is then introduced and planted within the solid materials-filled basin.
The most popularly used wetland plants to date are: bulrushes, cattails, reeds and sedges. Wetland plants, as defined and referred to in this invention, are any higher order of aquatic plant which have no natural ability to float in water, but must be rooted in soil or a solid substrate. Natural floating plants such as pond weeds, hyacinths, duckweeds and water lettuce which are not required to be rooted in soil or another solid substrate are not within the purview of this invention. This list is not intended by any means to be a complete one.
The usual method is to permit sufficient soil to remain attached to the wetland plant when it is planted within the solids filled basin to function as an initial nutrient and life support system for the plant. Waste water is then admitted into the basin after having first received primary treatment. The primary treatment is performed within a separate basin or tank large enough to allow for detention time of two to three hours, during which time most of the settleable solids will have settled out of the waste water onto the bottom of the primary tank for removal and treatment elsewhere. Such treatment may consist of aerobic or anaerobic digester facilities.
The removal of the settleable solids out of the waste water stream is now absolutely essential under the present technology, as their presence in the waste water stream, if admitted directly into the wetland treatment basin, will cause the solids to be deposited and piled up at the admitting end of the basin, filling up the spaces within the solids media of the basin causing odor, treatment and flow through problems. A flow through simply means that the water stream instead of filtering its way through the media and around the wetland roots where it may be treated, is prevented entry therein by the presence of settled solids, and simply overflows across the top of the basin and exits from the system in an untreated state.
Treatment of polluted water by wetland plants is accomplished because of the ability of wetland plants to bleed or leak oxygen from its root system into the surrounding water thereby supporting bacteria colonies which ingest and feed upon the nutrients and solids in the waste water stream. The level of the waste water presently admitted into the basin does not exceed the height of the solids media. Thus, as the relatively solids-free waste water filters through the solids media, it comes into contact with the root system of the wetland plants.
Wetland plants are morphologically and anatomically adapted to growing in a water saturated substrate by the presence of internal gas spaces called aerenchymas throughout the plant's tissue. The function of the aerenchyma is to supply oxygen to the buried or submerged root and other plant parts. The root and rhizomes, however, leak oxygen into the substrate creating oxidized and anoxic zones around the roots. Rhizomes, as referred to here, are roots and those portions of the root system that can send out new growth from the plant.
The presence of these oxidized and anoxic zones around the root systems creates a good environment for aerobic and facultative microorganisms in the rhizospheres. The aerobic bacteria consume and reduce the suspended solids as well as other substances in waste water that contribute to the reduction of oxygen in water referred to in the industry as Biological Oxygen Demand (BOD). The conversion of ammonia compounds to nitrate compounds is a function of these bacteria. A short distance away from the root or rhizome area lies the anoxic zone, wherein oxygen is in short supply. Still yet a further distance away lies the anaerobic zone. In these areas the facultative and anaerobic bacteria thrive, these bringing about the further reduction of suspended solids as well as the reduction of nitrate compounds to nitrogen gas and more basic substances. In short, each plant is a waste water treatment facility as it replicates in a small fashion precisely the functions of our multimillion dollar treatment plants with large concrete basins, pumps, air blowers, settling basins, chemicals, electric motors, lab facilities, etc.
Each plant is presently planted within the solids media basin with rich soil or humus around its root system to provide it with an initial support. As stated above, sometimes the solids media employed in the basin is soil itself. The presence of the soil is also important in the purification process. It removes suspended solids in the waste water, pathogenic bacteria and viruses, by filtration and sorption. Nutrients are removed from water flowing through soil in several ways ions such as NH.sub.4.sup.+, K.sup.+, and anions such as PO.sub.4.sup.+3 may be sorbed onto charged surfaces of humic substances. Organic and clay minerals have a higher exchange capability than coarsely textured mineral substances, such as gravel, and sand. This sorption process is not a permanent removal mechanism, but it buffers and stabilizes the system. However, precipitation and co-precipitation taking place within the soils is a more permanent treatment process. Heavy metals may be precipitated with sulfide and co-precipitation of phosphates with iron, aluminum and calcium can remove significant quantities of phosphorus, toxic substances such as heavy metals and radioactive materials are accumulated in the soil and this factor may determine the life of the soil as a media for the plant. Despite the aforementioned advantages, however, using a media comprised entirely of soil shares the same inherent disadvantage as using any other form of solids in the basin, as the entire basin volume becomes filled with soil, the space available to waste water therefore becomes limited and as the spaces between the soil particles become filled with waste and solids from the polluted stream, the plant is denied access to that stream until its root system expands to open up new avenues for the waste water stream. However, there is an upper limit to the growth of any plant root system, and eventually a clogging takes place within the soil media, necessitating the removal of both the plants and the entire soil media within the basin.
The wetland plants planted in the solids filled media basin are usually planted one to three feet apart. Under the best of conditions of temperature, food and plant propagation, it requires at the present time one acre of ground to treat 60,000 gallons of waste water per day. Where freezing temperatures occur seasonally, the treatment rate is reduced to approximately 40,000 gallons of polluted water per day. Removal rates fluctuate between 80% to 95%. Thus, to treat great quantities of waste water as from municipalities, and large industries, may well require treatment capability of one, two or several million gallons of waste water per day. Land demands and acreage requirements using the wetland treatment method as now developed would be quite large, and treatment effectiveness a limiting factor.
Another factor affecting the ability of the present system to treat waste water is the presence of settleable solids which normally range from 100 to 250 parts per million. Some form of pretreatment is presently utilized to remove these solids prior to the admission of waste water into the wetland treatment basin. Such pretreatment at the present time consists of building a large enough holding tank that will permit the waste water stream to be detained within the holding tank for a period of two to three hours. During that quiescent period, the settleable solids will settle to the bottom where they must then be removed for further treatment elsewhere.
Should no pretreatment be undertaken, then the solids in the waste water stream will fill the spaces between the media solids in the wetland basin, particularly at the front end of the basin, creating odor problems, restricting passage of the waste water to and around the root system of the plants, thereby limiting the effectiveness of the treatment process and causing a flow through of the waste water stream as henceforth described.
As the solids media within the basin occupies 80% to 90% of the total basin volume, in addition to limiting the quantity of waste water within the basin, also restricts the growth of the root system of the wetland plant. It is, after all, the root and rhizome area of the wetland plant that supplies oxygen to the bacteria, protozoa, and metazoa colonies that are responsible for the treatment process itself, and reduction of polluted matter within the waste water stream to gases and mineral matter. Further, the solids media presently employed is insignificant in the treatment process with the exception in those instances where soil itself is used a media throughout the basin. However, using soil as a media within the basin presents problems in itself. When soil is used to occupy the entire basin, it tends to compact and, although with the passage of time, the root system of the plants will create underground passages to permit the waste stream to interact with the rhizome area, there is still a tendency to encounter flow through problems.
Yet despite the limitations under which wetland plants presently have to function, they demonstrate a remarkable ability to treat industrial and/or municipal polluted water even in the cold climates of Canada and Northern Europe. These successes have developed despite the fact that the basins in which these wetland plants have been grown limit and restrict the plants' ability to reach and more effectively treat waste water. The effectiveness of the present technology nevertheless is brought out in the following instances.
A report by the Ontario Ministry of Environment presented in the 1981 Conference of Water Pollution Control Federation at Detroit, Mich. describes a one quarter acre open marsh in Listowel, Ontario, Canada planted in Cattail, Typha spp. The basin contained a solids media through out composed of subsoil, topsoil and 10% peat. Waste water was then allowed to flow into the marsh at a depth of 14 centimeters.
The rate of flow from Jul. 1 to Dec. 6, 1980 was at the rate of 4000 gallons per day. The results were, despite the physical limitations alluded to, impressive. In terms of mass loading and percent reduction, BOD was reduced 83% suspended solids were reduced 71% total phosphorus 89% NH.sub.3 reduced 99%, and total nitrogen is reduced 78%. This was achieved without artificial or mechanical introduction of air into the basin, in an open marsh and exposed to the rigors of a Canadian fall season. During the winter months, influent temperature of the water dropped to 35 degrees Fahrenheit, the severity of the cold caused almost total anoxia within the basin, pointing out the need of cover or hothousing of the basin area in the cold climates to keep the waste water temperature at a higher level.
Wetland treatment of waste water has been successfully employed in the U.S. at Iselin, Pa. A wetland basin using solids media of 4" stone and 16" sand with a growth of Cattails, has been successfully employed to treat waste water, at an application rate of 50,000 gallons per acre per day. An analysis and report of this facility by the Term Valley Authority, July, 1986, stated that economically marsh land treatment facility could be constructed at $2.00 per gallon per day of treatment capacity as compared to $3.00 to $15.00 per gallon per day for standard treatment facility. On page 10 of the report, the Tenn Valley Authority scientist reflecting on the removal efficiencies of fecal coliform attributed it to the effect of the root excretions of the plants. The report further added "These results are of special significance since they indicate artificial wetland system can be designed to either eliminate the need for chlorination or minimize the chlorine dosage needed to meet the permit limits." Despite the fact that the marsh facility was operated at or over its designed hydraulic load, it was able to reduce the fecal coliform count of 1,000,000 colonies per 100 ml to 720 or almost 100% per cent reduction.
A comparison on the effectiveness of the various types of wetland plants which the author refers to as the higher forms of aquatic plants was given in a report by Gersberg, Elkins, Lyon, Goldman, San Diego Region Reclamation Agency and division of Environment studies, University of California 1985. This was a qualitative assessment on primary waste water treatment at the Santee Water Reclamation facility, Santee, Calif. Four basins were constructed, each 18.5 meters long, 3.5 meters wide and 0.76 meters deep. The basins were plastic lined and filled with gravel. Three of the beds were either planted in bulrushes, reeds or cattails and the fourth served as an unvegetated control bed. Mean concentrations of the primary waste water was 27.8 mg/l total (N), 24.7 mg/l ammonia (N), BOD was 118.3 mg/l and suspended solids, 57.3 mg/l total (N), 24.7 mg/l. Inflow to each bed was 1210 gallons per day or a hydraulic load at the rate of 77,129 gallons per acre per day. Residence time was 6 days. The test period extended from August 1983 to December 1984.
The basin containing bulrushes proved the most effective of all the wetland plants employed in reducing pollutants. Ammonia (N) was reduced from 24.7 mg/l.+-.5.8 mg/l to 1.5 mg/l.+-.1.7 mg/l. The author said, on page 365 volume 20, No. 3 Wat Res, Great Britain, "that when sufficient dissolved carbon is present, the artificial wetlands are very efficient at promoting denitrification, with removal efficiencies of greater than 95% of the nitrate present in secondary waste water at application rates of 40 cm a day." This would be almost 10 times the rate applied in the instant case. The higher nitrogen removal was attributable by the researchers to the ability of the bulrushes, cattails and reeds to transport oxygen down to the roots, and 28% by cattails and only 11% by the unvegetated bed. It was felt that the extended root system of the bulrushes and reeds, greater than 60 cm. as compared to 30 cm. for the cattails, was responsible for the superior removal rates of ammonia and BOD among the former. Mean BOD removal efficiencies (relative to inflow) were 96%, 81.5%, 74% and 69% for the bulrushes, reeds, cattails and unvegetated beds, respectively. The author said, "Since the B.O.D. removal (organic compound degradation) is enhanced under aerobic degradation, it is reasonable to assume that the superior treatment afforded by the Bulrush was due to plant translocation of oxygen to an otherwise anaerobic area." It was stated further on page 367, "Other investigators, however have found artificial wetlands are well suited for waste water treatment even in moderately cold climates, such as Ontario, Canada where they can be operated year round" (Wile 1982 Reed at 1984).
In concluding, the authors reflected on the satisfactory substitution of wetland basins for standard municipal plants, stating on page 365, "The Bulrush bed at the hydraulic application rate of 4.7 meters cubed per day (77,129 gallons per day) produced an effluent with B.O.D. and Total Suspended Solids values less that 10/10 mg/l, standard for advanced secondary treatment and a total nitrogen level less than 2 mg/l. At this waste water application rate approximately 20 acres of constructed wetlands would be required to treat one million gallons per day."
The present invention is designed to eliminate practically all media from the wetland basin, thus permitting unrestricted growth of the plant root and rhizome system, greatly reducing the land area needed to treat comparable flow of waste water to approximately one fourth to one fifth the size presently required in existing systems. Thus, less land will treat more waste water, making this an attractive and cost effective means of treating municipal and industrial waste water in quantities of one million gallons per day and more. It eliminates the necessity of pretreatment as the entire floor of the basin can now be utilized as a receiver for the settleable solids, as the solids media that once occupied the entire basin, is now absent. The absence of solid media means that raw waste water can be directly admitted into the basin without the necessity of building a primary tank. The size or floor area of the wetland basin is on the order of 100 times greater than that of a primary tank. Hence, it is possible that this greatly enlarged floor area of the basin can be utilized to reduce or degrade settled solids through anaerobic and facultative bacterial action whereas by contrast, the floor area of the primary tank being smaller and not designed for treatment but only for collection purposes, is an expensive necessity when a basin for wetland basin is filled with solid media, but is not a necessity when a basin for wetland plants can be designed and operated without such a solid media. The manner in which treatment of settled solids in the wetland basin will be undertaken, shall be explored further in more detail in the Description of the Invention. As a great deal of the settled solids are treated and reduced on the wetland basin floor, further treatment and reduction facilities, such as anaerobic or aerobic digesters can be downsized to less than half in size and cost as those that would otherwise be required using the present technology.